Copper-containing, metallic effect pigments comprising copper pigments or brass pigments produced from a copper-zinc alloy, and also referred to as gold bronze pigments, are used in industries including the graphics industry among others, such as in printing inks, for example.
The metallic effect pigments produced from copper or brass powder by milling (conventionally predominantly in a dry milling operation) and used today for the pigmentation of flexographic and gravure inks, such as, for example, “Rotovario” gold bronze pigment dispersions or “Rotoflex” stabilized leafing gold bronze pigment powder or “Rotosafe” stabilized leafing gold bronze pigment pellets from Eckart GmbH, D-90763 Fürth, Germany, have leafing properties which impose conditions on their suitability for the pigmenting of film reverse applications with mirrorlike effect.
The use of brass pigments produced by PVD methods in printing inks is also problematic in that homogeneous metallization of the two metals (copper and zinc) for the purpose of achieving a uniform hue is extremely difficult technically to accomplish, with very different vaporization temperatures, in a high vacuum. Moreover, the PVD brass pigments, which are relatively expensive to produce, differ from brass pigments produced by conventional milling in having not very compact layers, with densities below the densities of the respective materials, and the desired gold hues (especially rich gold) cannot be realized at the low film thicknesses desired.
EP 1 529 084 B1 describes gold bronze pigments which can be produced by PVD methods. On account of the complex method, these pigments are very expensive. Moreover, these pigments have a propensity toward partial phase separation of the alloy constituents, a phenomenon likewise accompanied by unwanted shifts in hue and inadequate hue stabilities.
In addition, no copper-containing effect pigments produced by PVD methods have been available commercially to date.
The PVD aluminum pigments that are therefore normally used for the pigmentation of film reverse applications with mirrorlike effect, such as the “Metalure” products from Eckart GmbH, for example, do feature a homogeneous surface nature with a perfect nonleafing behavior, but have to be color-tinted with yellowish toner pigments. Printing inks of this kind trade under the product name “Ultrastar” from Eckart, for example, as solventborne gravure and flexographic inks. On account of the complex mode of production, PVD metallic effect pigments are generally much more expensive than metallic effect pigments produced by means of milling, conventionally.
In reverse application, as it is called, where a transparent film is printed with the printing ink, these printing inks can be used to produce a virtually perfect metal mirror. The mirror, however, is visible only if the application is viewed from the film side. Because of the admixed yellowish toner, the metal mirror appears in golden hues. The silver luster of the aluminum pigments, accordingly, mixes with the inherent color of the colorants.
A disadvantage of these printing inks is that their color is not very intense. The PVD aluminum pigments tend to accumulate in the vicinity of the film, i.e., in the lower region of the printed film. Because of this, however, there is little color pigment present between aluminum pigments and film, and this lessens the color strength.
If the concentration of color pigment is increased for compensation, corresponding luster losses are the inevitable consequence.
The printing ink, furthermore, has disadvantages when applied to absorbent substrates such as paper, for example, on account of the possibility for separation between metallic pigment and color pigment.
In order to produce conventional brass pigments produced from brass powder by milling, high-purity, electrolytically obtained copper and zinc are used as starting material for the milling operation, and are alloyed with addition of a little aluminum as reducing agent. For this purpose, copper and zinc are melted with one another, and the brass melt produced is atomized to form a coarse, nodular brass powder. The brass powder obtained is then ground into brass flakes. The milling of brass powder is carried out predominantly by the Hametag dry milling process. In this process, the coarse brass powder is ground in ball mills in a number of milling stages under different milling conditions, such as mill size, mill diameter, rotational mill velocity, ball size, and milling time, for example, with addition of lubricant, such as stearic or oleic acid, for example, in order to prevent cold welding of the copper or brass particles, and with grinding assistants, such as steel balls, for example.
For the dry milling of brass powder, the coarse, nodular brass powder used as milling product is ground to form platelet-shaped brass pigments. The density of the brass platelets, which are relatively difficult to deform, is around three times as high as that of comparable aluminum platelets. Following milling and classifying, the brass pigments are collected in different containers and then homogenized. In order to give the subsequent, metallically pigmented coatings the requisite metallic luster, it is possible, during the subsequent aftertreatment, for additional additives (such as stearic acid, for example) to be “polished on” to the surface of the pigment platelets.
In the case of brass pigments (gold bronze pigments), the hue of the alloy is determined by the ratio of copper to zinc. Generally speaking, the copper content is between 70% and 100% by weight. Gold bronze pigments trade, in characteristic natural hues, as “pale gold” with a copper fraction of around 90% by weight, remainder zinc; as “rich pale gold” with a copper fraction of around 85% by weight, remainder zinc; and as “rich gold” with a copper fraction of around 70% by weight, remainder zinc.
The production of platelet-shaped brass pigments by milling of brass powder in the presence of grinding auxiliaries is known to a person skilled in the art of pigment production, and is described in DE 2007717 A, for example. In this process for producing brass pigments, using a wet milling operation carried out with an inert liquid, the ground product is separated into at least three particle size fractions, i.e., into two coarse fractions and one fine fraction having a particle size of less than 44 μm. The fine fraction produced is recovered, and the middle coarse fraction is removed from the operation, while at least one coarse fraction is recycled with the oversize to the mill (ball mill).
JP 63000406A shows a process for the simple and cost-effective production of metal powder. The powder particles consist of metal flakes produced by mechanical milling, in ball mills, for example, using oil and water, including, for example, flakes of brass, having a high form factor and a normal pigment diameter and thickness.
JP 2002327201A relates to a golden powder for dip coating, composed of brass flakes having an average pigment diameter of 40-60 μm, a bulk density of 0.5-0.7 g/cm3, and a covering area of at least 7000 cm2/g.
U.S. Pat. No. 2,002,891A relates to the production of a bronze powder from aluminum, copper or other metals and their alloys. The metal used is milled under defined milling conditions to form flake-, platelet- or scale-shaped powder particles.
U.S. Pat. No. 3,995,815 A describes a process for producing metal powder comprising flakes, by means of a wet milling operation, defined by mixing ratios and milling times, and carried out on metals in ball mills. No details of the milled products are given by this publication.
U.S. Pat. No. 4,172,720 A discloses a flake-shaped metal powder with narrow thickness distribution, pure color, and a very high mirror effect. This known metal powder is produced from metal, including, for example, from brass, by a wet milling operation characterized by particular weight ratios of grinding assistant, metal, lubricant, and milling liquid.
Copper flakes with a thickness of below 3 μm and a diameter below 10 μm can be produced by the process described in U.S. Pat. No. 4,884,754. In that process, copper particles are milled in a nonpolar organic solvent, in the presence of one or more organic lubricants.
Subsequently the main fraction of organic lubricant and solvent is removed, and the copper flakes obtained are milled again, in a jet mill.
Copper flakes having a diameter of 4 to 10 μm and a form factor of 2 to 20 for application in electrically conductive pastes, for example, are described in US 2002/0050186 A1.
Thin, platelet-shaped aluminum pigments with a narrow thickness distribution are known from WO 2008/077612 A2. These aluminum pigments have a thickness h50, determined via thickness counting by scanning electron microscopy, of 15 to 75 nm.
The high market demands that exists for printing inks, especially for gravure and flexographic inks for producing gold-colored mirrorlike effects cannot at present be satisfied.